Low phosphorus lubricating oil composition having lead corrosion control

ABSTRACT

The present invention provides a low phosphorus lubricating oil composition containing a mixture of zinc dithiophosphates in a certain ratio surprisingly yields improved lead corrosion. The synergistic combination of mixed zinc dithiophosphates containing a zinc primary dialkyl dithiophosphate, zinc secondary dialkyl dithiophosphate and zinc diaryl dithiophosphate in a respective ratio, based on the phosphorus content, of the zinc primary dialkyl dithiphosphate to zinc secondary dialkyl dithiophosphate from about 2:1 to about 1:2 and the ratio of the mixture of zinc primary dialkyl dithiophosphate and zinc secondary dialkyl dithiophosphate to zinc diaryl dithiophosphate from about 6:1 to about 1:1. When used in a lubricating oil composition having a total phosphorus content less than about 0.06 wt %, based on the total weight of the lubricating oil composition to lubricate internal combustion engines. The mixture of zinc dithiophosphates greatly reduces lead corrosion.

The present invention is related, in part, to a lubricating oilcomposition. More particularly, the present invention relates to a lowphosphorus lubricating oil composition employing a mixture of zincdithiophosphates and wherein the lubricating oil composition has lessthan about 0.06 wt % total phosphorus content based on the total weightof the lubricating oil composition. The low phosphorus lubricating oilcomposition of the present invention is effective in lead corrosioncontrol when used as a lubricating oil composition in internalcombustion engines.

BACKGROUND OF THE INVENTION

Emissions arising from automotive exhaust has been a problem for severaldecades and approaches for addressing this problem have included the useof unleaded fuel (to deal, in part, with lead pollution arising fromleaded fuels), oxygenated fuel (to reduce hydrocarbon emissions), theuse of catalytic converters (also to reduce hydrocarbon emissions), etc.

Catalytic converters are now universally employed with gasoline poweredvehicles and the efficiency of these converters is directly related tothe ability of the catalyst to effect conversion of unburnt or partiallyburnt hydrocarbons generated during combustion to carbon dioxide andwater. One problem arising with the use of such converters is poisoningof the catalyst resulting in reduced catalyst efficiency. Sincecatalytic converters are intended for extended use, catalyst poisoningresults in higher levels of atmospheric discharges of pollutants frominternal combustion engines over prolonged periods of time.

In order to minimize such poisoning, the industry has set standards forboth fuel and lubricant contents. For example, standards for fuels haveincluded the use of unleaded gasoline in order to avoid lead poisoningof the catalyst as well as lead discharge into the environment. See, forexample, Buckley, III, “Long Chain Aliphatic Hydrocarbyl Amine AdditivesHaving an Oxyalkylene Hydroxy Connecting Group”, U.S. Pat. No.4,975,096, issued Dec. 4, 1990.

As to the lubricants, one additive family currently being addressed byindustry standards is the phosphorus-containing additives such as zincdithiophosphate wear inhibitors used in lubricant compositions employedto lubricate internal combustion engines. Specifically,phosphorus-containing additives reach the catalytic converter as aresult of, for example, exhaust gas recirculation and/or oil blow-byprocesses as well as other methods known in the art. See, for example,Beck, et al. “Impact of Oil-Derived Catalyst Poisons on FTP Performanceof LEV Catalyst Systems”, SAE Technical Paper 972842 (1997) and Darr etal. “Effects of Oil-Derived Contaminants on Emissions from TWC-EquippedVehicles”, SAE 2000-01-1881 (2000). In any event, the phosphorus isknown to accumulate in the catalytic converter, at active metal sites;thus reducing catalyst efficiency and effectively over time, poisoningthe catalyst. As a result of the above, a new focus is to lowerphosphorus in the lubricating oils. For example, the draft GF-4specifications for lubricant compositions have proposed significantlylower phosphorus contents than heretofore employed.

A problem arises when the level of phosphorus is reduced in a lubricantcomposition containing an oil-soluble, phosphorus-containing, anti-wearcompound in that there is a significant reduction in anti-wearperformance arising from this diminution in phosphorus content, One wellknown class of antiwear additives are metal alkylphosphates, especiallyzinc dialkyl dithiophosphates, are generally employed in lubricatingoils at phosphorous levels above 0.1 weight percent when used for wearcontrol, At lower levels, it is not found to be an effective antiwearadditive. For instance, as exemplified in U.S. Pat. No. 6,696,393,issued Feb. 24, 2004, lowering the level of phosphorus due to thepresence of a metal dithiophosphate additive in a lubricant compositionby one-half from 0.095 weight percent to 0.048 weight percent phosphorusresults in about a seven-fold increase in engine wear.

Zinc dithiophosphates have either dialkyl or diaryl groups. Zinc dialkyldithiophosphates are further subdivided into primary alkyl and secondaryalkyl zinc dithiophosphates. Pentan-1-ol and 3-methylbutan-2-ol areillustrative of the primary and secondary alcohols used to prepareprimary and secondary zinc dithiophosphates. Different zincdithiophosphate chemical types perform differently (See below).

Performance Parameters of Zinc Dithiophosphate Types Primary AlkylSecondary Alkyl Aryl Thermal Stability Medium Low High AntiwearProtection Medium High Low Hydrolytic Stability Medium High Low

Each type has important applications in modern additive packages. It istherefore important to have the right mix of zinc dithiophosphates inany given lubricating oil composition to provide adequate anti-wearperformance and at the same time keeping the phosphorus levels, due tothe presence of a metal dithiophosphate additive, below 0.1 wt % becausephosphorus has a tendency to accumulate in the catalytic converter thusreducing catalyst efficiency, poisoning the catalyst.

SUMMARY OF THE INVENTION

As previously mentioned, the present invention is related, in part, to alubricating oil composition. More particularly, the present inventionrelates to a low phosphorus lubricating oil composition employing amixture of zinc dithiophosphates in a certain ratio and wherein thelubricating oil composition has less than about 0.06 wt % totalphosphorus content, based on the total weight of the lubricating oilcomposition. The low phosphorus lubricating oil composition of thepresent invention is effective in lead corrosion control when used as alubricating oil composition in internal combustion engines.

Accordingly, in its broadest aspect, the present invention is related toa lubricating oil composition comprising a major amount of base oil oflubricating viscosity and a minor amount of a mixture of a zinc primarydialkyl dithiophosphate, a zinc secondary dialkyl dithiophosphate and azinc diaryl dithiophosphate wherein the respective ratio, based on thephosphorus content, of the zinc primary dialkyl dithiophosphate to zincsecondary dialkyl dithiophosphate is from about 2:1 to about 1:2 and theratio of the mixture of zinc primary dialkyl dithiophosphate and zincsecondary dialkyl dithiophosphate to zinc diaryl dithiophosphate is fromabout 6:1 to about 1:1 and wherein the total phosphorus content of thelubricating oil composition is less than about 0.06 wt %, based on thetotal weight of the lubricating oil composition.

The minor amount of the mixture of a zinc primary dialkyldithiophosphate, a zinc secondary dialkyl dithiophosphate and a zincdiaryl dithiophosphate employed in the lubricating oil composition ofthe present invention is from about 0.1 wt % to about 1.5 wt %,preferably from about 0.3 wt % to about 1.2% and more preferably about0.5 wt % to about 1.0 wt %, based on the total weight of the lubricatingoil composition.

The lubricating oil composition of the present invention will containfrom about 0.05 wt % to about 1.2 wt % of a zinc primary dialkyldithiophosphate, from about 0.05 wt % to about 1.2 wt % of a zincsecondary dialkyl dithiophosphate and from about 0.02 wt % to about 0.7wt % of a zinc diaryl dithiophosphate, based on the total weight of thelubricating oil composition. Preferably, the lubricating oil compositionof the present invention will contain from about 0.1 wt % to about 0.7wt % of a zinc primary dialkyl dithiophosphate, from about 0.1 wt % toabout 0.7 wt % of a zinc secondary dialkyl dithiophosphate and fromabout 0.05 wt % to about 0.5 wt % of a zinc diaryl dithiophosphate,based on the total weight of the lubricating oil composition. Morepreferably, the lubricating oil composition of the present inventionwill contain from about 0.2 wt % to about 0.5 wt % of a zinc primarydialkyl dithiophosphate, from about 0.2 wt % to about 0.5 wt % of a zincsecondary dialkyl dithiophosphate and from about 0.1 wt % to about 0.3wt % of a zinc diaryl dithiophosphate, based on the total weight of thelubricating oil composition.

The primary alkyl group of the zinc primary dialkyl dithiophosphate hasfrom about C₁ to about C₁₃ carbon atoms, preferably from about C₃ toabout C₁₀ carbon atoms and more preferably, from about C₆ to about C₈carbon atoms.

The secondary alkyl group of the zinc secondary dialkyl dithiophosphatehas from about C₃ to about C₁₃ carbon atoms, preferably from about C₃ toabout C₈ carbon atoms and more preferably, from about C₃ to about C₆carbon atoms.

The aryl group of the zinc diaryl dithiophosphate has from about C₆ toabout C₃₀ carbon atoms, preferably from about C₆ to about C₂₄ carbonatoms and more preferably, from about C₆ to about C₂₀ carbon atoms.

In a preferred embodiment, the respective ratio, based on the phosphoruscontent, of zinc primary dialkyl dithiophosphate to zinc secondarydialkyl dithiophosphate is from about 3:2 to about 2:3. More preferably,the ratio is about 1:1.

In a preferred embodiment, the respective ratio, based on the phosphoruscontent, of the mixture of zinc primary dialkyl dithiophosphate and zincsecondary dialkyl dithiophosphate to zinc diaryl dithiophosphate is fromabout 4:1 to about 1:1. More preferably, the ratio is about 2:1.

In an especially preferred embodiment, the respective ratio, based onthe phosphorus content, of the mixture of zinc primary dialkyldithiophosphate to zinc secondary dialkyl dithiophosphate to zinc diaryldithiophosphate is 1:1:1.

In another embodiment, the total phosphorus content in the lubricatingoil composition of the present invention is preferably less than about0.05 wt %, more preferably, based on the total weight of the lubricatingoil composition.

In still another embodiment, the sulfur content in the lubricating oilcomposition of the present invention is less than about 0.5 wt % and,preferably, less than about 0.2 wt %, based on the total weight of thelubricating oil composition and the total sulfated ash content in thelubricating oil composition of the present invention is less than about1.2 wt %, preferably, less than about 1.0 wt %, and more preferably lessthan about 0.8 wt %, based on the total weight of the lubricating oilcomposition.

In one of its method aspects, the present invention further relates to amethod for improving lead corrosion. The method involves operating aninternal combustion engine with a lubricating oil composition comprisinga major amount of base oil of lubricating viscosity and a minor amountof a mixture of a zinc primary dialkyl dithiophosphate, a zinc secondarydialkyl dithiophosphate and a zinc diaryl dithiophosphate wherein therespective ratio, based on the phosphorus content, of the zinc primarydialkyl dithiophosphate to zinc secondary dialkyl dithiophosphate isfrom about 2:1 to about 1:2 and the ratio of the mixture of zinc primarydialkyl dithiophosphate and zinc secondary dialkyl dithiophosphate tozinc diaryl dithiophosphate is from about 6:1 to about 1:1 and whereinthe total phosphorus content of the lubricating oil composition is lessthan about 0.06 wt %, based on the total weight of the lubricating oilcomposition.

Among other factors, the present invention provides a low phosphoruslubricating oil composition containing a mixture of zincdithiophosphates in a certain ratio surprisingly yields improved leadcorrosion. The mixture of zinc dithiophospates contains a zinc primarydialkyl dithiophosphate, a zinc secondary dialkyl dithiophosphate and azinc diaryl dithiophosphate. The synergistic combination of mixed zincdithiophosphates wherein the respective ratio, based on the phosphoruscontent, of the zinc primary dialkyl dithiophosphate to zinc secondarydialkyl dithiophosphate is from about 2:1 to about 1:2 and the ratio ofthe mixture of zinc primary dialkyl dithiophosphate and zinc secondarydialkyl dithiophosphate to zinc diaryl dithiophosphate is from about 6:1to about 1:1 when used in a lubricating oil composition having a totalphosphorus content less than about 0.06 wt %, based on the total weightof the lubricating oil composition, greatly reduces lead corrosion whenused to lubricate internal combustion engines.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is related, in part, to a lubricating oilcomposition, More particularly, the present invention relates to a lowphosphorus lubricating oil composition employing a mixture of zincdithiophosphates containing a zinc primary dialkyl dithiophosphate, azinc secondary dialkyl dithiophosphate and a zinc diaryl dithiophosphatewherein the respective ratio, based on the phosphorus content, of thezinc primary dialkyl dithiophosphate to zinc secondary dialkyldithiophosphate is from about 2:1 to about 1:2 and the ratio of themixture of zinc primary dialkyl dithiophosphate and zinc secondarydialkyl dithiophosphate to zinc diaryl dithiophosphate is from about 6:1to about 1:1 and wherein the lubricating oil composition has less thanabout 0.06 wt % total phosphorus content, based on the total weight ofthe lubricating oil composition. The low phosphorus lubricating oilcomposition of the present invention is effective in lead corrosioncontrol when used as a lubricating oil in internal combustion engines.

Each of these components in the claimed composition will be described indetail herein. However, prior to such a description, the following termswill first be defined.

The term “alkyl” refers to both straight- and branched-chain alkylgroups.

The term “aryl” refers to a substituted or unsubstituted aromatic group,such as the phenyl, tolyl, xylyl, ethylphenyl and cumenyl groups.

The term “low phosphorus” refers to the phosphorus content of thelubricating oil composition of the present invention. The phosphoruscontent is in the range of about 0.005 weight percent to about 0.06weight percent based on the total weight of the lubricating oilcomposition.

The term “total phosphorus” refers to the total amount of phosphorus inthe lubricant composition regardless of whether such phosphorus ispresent as part of an oil-soluble, phosphorus-containing, anti-wearcompound or in the form of a contaminant in the lubricant compositionsuch as residual phosphorus remaining due to the presence of P₂S₅ usedto prepare metal dihydrocarbyl dithiophosphates. In either event, theamount of phosphorus permitted in the lubricant composition isindependent of source. Preferably, however, the phosphorus is part of alubricant additive.

Unless otherwise specified, all percentages are in weight percent.

The Zinc Dithiophosphate Compound

The lubricating oil composition of the present invention will employ, inpart, a mixture of zinc dithiophosphates. The zinc dithiophosphates areindependently characterized by formula I:

wherein each R is independently a group containing from about 1 to about30 carbon atoms.

The R groups in the dithiophosphate can independently be about C₁ toabout C₁₃ primary alkyl, about C₃ to about C₁₃ secondary alkyl, andabout C₆ to about C₃₀ aryl group. Preferably, the R groups in thedithiophosphate can independently be about C₃ to about C₁₀ primaryalkyl, about C₃ to about C₆ secondary alkyl, and about C₆ to about C₂₄aryl group. More preferably, the R groups in the dithiophosphate canindependently be about C₆ to about C₈ primary alkyl, about C₃ to aboutC₆ secondary alkyl, and about C₆ to about C₂₀ aryl group. The R groupsmay be a substantially hydrocarbon group. By “substantially hydrocarbon”is meant hydrocarbons that contain substituent groups such as ether,ester, nitro, or halogen which do not materially affect the hydrocarboncharacter of the group.

The R group of the zinc dithiophosphate may be derived, for example,from a primary alcohol such as methanol, ethanol, propanol, butanol,pentanol, hexanol, heptanol, octanol, nonanol, decanol, dodecanol,octadecanol, propenol, butenol, 2-ethylhexanol; a secondary alcohol suchas isopropyl alcohol, secondary butyl alcohol, isobutanol,3-methylbutan-2-ol, 2-pentanol, 4-methyl-2-pentanol, 2-hexanol,3-hexanol, amyl alcohol; an aryl alcohol such as phenol, substitutedphenol (particularly alkylphenol such as butylphenol, octylphenol,nonylphenol, dodecylphenol), disubstituted phenol.

Preferably the R group will be independently a primary alkyl, asecondary alkyl or an aryl group,

For the present invention it is contemplated that the mixture of a zincprimary dialkyl dithiophosphate, a zinc secondary dialkyldithiophosphate and a zinc diaryl dithiophosphate will be in arespective ratio, based on the phosphorus content, in the lubricatingoil composition of the present invention. The ratio of zinc primarydialkyl dithiophosphate to zinc secondary dialkyl dithiophosphate willbe from about 2:1 to about 1:2 and the ratio of the mixture of zincprimary dialkyl dithiophosphate and zinc secondary dialkyldithiophosphate to zinc diaryl dithiophosphate is from about 5:1 toabout 1:1. Preferably, the respective ratio, based on the phosphoruscontent, of zinc primary dialkyl dithiophosphate to zinc secondarydialkyl dithiophosphate is a range from about 3:2 to about 2:3, morepreferably about 1:1. Preferably, the respective ratio, based on thephosphorus content, of the mixture of zinc primary dialkyldithiophosphate and zinc secondary dialkyl dithiophosphate to zincdiaryl dithiophosphate is a range from about 4:1 to about 1:1, morepreferably about 2:1. Most preferably, the respective ratio, based onthe phosphorus content, of the mixture of zinc primary dialkyldithiophosphate to zinc secondary dialkyl dithiophosphate to zinc diaryldithiophosphate is 1:1:1.

Many of the zinc dithiophosphates useful in the present invention areavailable commercially. However, zinc dithiophosphates are widely knownin the art and a skilled artisan can readily synthesize such compoundsfor the purpose of the present invention. Typically, zincdithiophosphates can be made by initial reaction of phosphorouspentasulfide and an alcohol or phenol or mixtures of alcohols and/orphenols such as those illustrated above for the R group, The reactioninvolves four moles of the alcohol or phenol per mole of phosphorouspentasulfide, and may be carried out within the temperature range fromabout 50° C. to about 200° C. Thus, the preparation of O,O-di-n-hexylphosphorodithioic acid, for example, involves the reaction ofphosphorous pentasulfide with four moles of n-hexyl alcohol at about100° C. for about two hours. Hydrogen sulfide is liberated and theresidue is phosphorodithioic acid. The preparation of the metal salt ofthis acid may be effected by reaction with either zinc oxide or zinchydroxide to yield the zinc dithiophosphate. Simply mixing and heatingthese two reactants is sufficient to cause the reaction to take placeand the resulting product is sufficiently pure for the purposes of thepresent invention.

Patents describing the synthesis of such zinc dithiophosphates includeU.S. Pat. Nos. 2,680,123; 3,000,822; 3,151,075; 3,385,791; 4,377,527;4,495,075 and 4,778,906. Each of these patents is incorporated herein byreference in their entirety.

The Lubricating Oil Composition

The mixture of zinc dithiophosphates of the present invention istypically added to a base oil in sufficient amounts to provide leadcorrosion control in internal combustion engines. Generally, thelubricating oil composition of the present invention will contain amajor amount of base oil of lubricating viscosity and a minor amount ofthe mixture of zinc dithiophosphates of the present invention.

Base Oil of Lubricating Viscosity

Base oil as used herein is defined as a base stock or blend of basestocks which is a lubricant component that is produced by eachmanufacturer to the same specifications (independent of feed source ormanufacturers location); that meets the same manufacturer'sspecification; and that is identified by a unique formula, productidentification number, or both. Base stocks may be manufactured using avariety of different processes including but not limited todistillation, solvent refining, hydrogen processing, oligomerization,esterification, and rerefining.

Rerefined stock shall be substantially free from materials introducedthrough manufacturing, contamination, or previous use. The base oil ofthis invention may be any natural or synthetic lubricating base oilfraction particularly those having a kinematic viscosity at 100°Centigrade (° C.) and about 4 centistokes (cSt) to about 20 cSt.Hydrocarbon synthetic oils may include, for example, oils prepared fromthe polymerization of ethylene, polyalphaolefin or PAO, or fromhydrocarbon synthesis procedures using carbon monoxide and hydrogengases such as in a Fisher-Tropsch process. A preferred base oil is onethat comprises little, if any, heavy fraction; e.g., little, if any,lube oil fraction of viscosity about 20 cSt or higher at about 100° C.Oils used as the base oil will be selected or blended depending on thedesired end use and the additives in the finished oil to give thedesired grade of engine oil, e.g. a lubricating oil composition havingan SAE Viscosity Grade of 0W, 0W-20, 0W-30, 0W-40, 0W-50, 0W-60, 5W,5W-20, 5W-30, 5W-40, 5W-50, 5W-60, 10W, 10W-20, 10W-30, 10W-40, 10W-50,15W, 15W-20, 15W-30, or 15W-40.

The base oil may be derived from natural lubricating oils, syntheticlubricating oils or mixtures thereof. Suitable base oil includes basestocks obtained by isomerization of synthetic wax and slack wax, as wellas hydrocrackate base stocks produced by hydrocracking (rather thansolvent extracting) the aromatic and polar components of the crude,Suitable base oils include those in all API categories I, II, III, IVand V as defined in API Publication 1509, 14th Edition, Addendum I,December 1998. Saturates levels and viscosity indices for Group I, IIand III base oils are listed in Table I. Group IV base oils arepolyalphaolefins (PAO), Group V base oils include all other base oilsnot included in Group I, II, III, or IV. Group III base oils arepreferred.

TABLE I SATURATES, SULFUR AND VISCOSITY INDEX OF GROUP I, II, III, IVAND V BASE STOCKS Saturates (As determined by ASTM D2007) ViscosityIndex Sulfur (As determined by (As determined by ASTM D4294, Group ASTMD2270) ASTM D4297 or ASTM D3120) I Less than 90% saturates and/orGreater than or equal to 80 and Greater than to 0.03% sulfur less than120 II Greater than or equal to 90% Greater than or equal to 80 andsaturates and less than or equal to less than 120 0.03% sulfur IIIGreater than or equal to 90% Greater than or equal to 120 saturates andless than or equal to 0.03% sulfur IV All Polyalphaolefins (PAOs) V Allothers not included in Groups I, II, III, or IV

Natural lubricating oils may include animal oils, vegetable oils (e.g.,rapeseed oils, castor oils and lard oil), petroleum oils, mineral oils,and oils derived from coal or shale.

Synthetic oils may include hydrocarbon oils and halo-substitutedhydrocarbon oils such as polymerized and inter-polymerized olefins,alkylbenzenes, polyphenyls, alkylated diphenyl ethers, alkylateddiphenyl sulfides, as well as their derivatives, analogues andhomologues thereof, and the like. Synthetic lubricating oils alsoinclude alkylene oxide polymers, interpolymers, copolymers andderivatives thereof wherein the terminal hydroxyl groups have beenmodified by esterification, etherification, etc. Another suitable classof synthetic lubricating oils comprises the esters of dicarboxylic acidswith a variety of alcohols. Esters useful as synthetic oils also includethose made from about C₅ to about C₁₂ monocarboxylic acids and polyolsand polyol ethers. Tri-alkyl phosphate ester oils such as thoseexemplified by tri-n-butyl phosphate and tri-iso-butyl phosphate arealso suitable for use as base oils.

Silicon-based oils such as the polyalkyl-, polyaryl-, polyalkoxy-, orpolyaryloxy-siloxane oils and silicate oils) comprise another usefulclass of synthetic lubricating oils. Other synthetic lubricating oilsinclude liquid esters of phosphorus-containing acids, polymerictetrahydrofurans, polyalphaolefins, and the like.

The base oil may be derived from unrefined, refined, rerefined oils, ormixtures thereof. Unrefined oils are obtained directly from a naturalsource or synthetic source (e.g., coal, shale, or tar sand bitumen)without further purification or treatment. Examples of unrefined oilsinclude a shale oil obtained directly from a retorting operation, apetroleum oil obtained directly from distillation, or an ester oilobtained directly from an esterification process, each of which may thenbe used without further treatment. Refined oils are similar to theunrefined oils except that refined oils have been treated in one or morepurification steps to improve one or more properties. Suitablepurification techniques include distillation, hydrocracking,hydrotreating, dewaxing, solvent extraction, acid or base extraction,filtration, and percolation, all of which are known to those skilled inthe art Rerefined oils are obtained by treating used oils in processessimilar to those used to obtain the refined oils. These rerefined oilsare also known as reclaimed or reprocessed oils and often areadditionally processed by techniques for removal of spent additives andoil breakdown products.

Base oil derived from the hydroisomerization of wax may also be used,either alone or in combination with the aforesaid natural and/orsynthetic base oil.

Such wax isomerate oil is produced by the hydroisomerization of naturalor synthetic waxes or mixtures thereof over a hydroisomerizationcatalyst.

It is preferred to use a major amount of base oil in the lubricating oilcomposition of the present invention. A major amount of base oil asdefined herein comprises about 40 wt % or more. Preferred amounts ofbase oil comprise about 40 wt % to about 97 wt %, preferably greaterthan about 50 wt % to about 97 wt %, more preferably about 60 wt % toabout 97 wt % and most preferably about 80 wt % to about 95 wt % of thelubricating oil composition. (When weight percent is used herein, it isreferring to weight percent of the lubricating oil unless otherwisespecified.)

The amount of the mixture of zinc dithiophosphates employed in thelubricating oil composition of the present invention will be in a minoramount compared to the base oil of lubricating viscosity. Generally, itwill be in an amount from about 0.1 wt % to about 1.5 wt %, preferablyfrom about 0.3 wt % to about 1.2 wt % and more preferably from about 0.5wt % to about 1.0 wt %, based on the total weight of the lubricating oilcomposition.

The lubricating oil composition of the present invention will containfrom about 0.05 wt % to about 1.2 wt %, preferably from about 0.1 wt %to about 0.7 wt %, and more preferably from about 0.2 wt % to about 0.5wt % of a zinc primary dialkyl dithiophosphate, based on the totalweight of the lubricating oil composition.

The lubricating oil composition of the present invention will containfrom about 0.05 wt % to about 1.2 wt %, preferably from about 0.1 wt %to about 0.7 wt %, and more preferably from about 0.2 wt % to about 0.5wt % of a zinc secondary dialkyl dithiophosphate, based on the totalweight of the lubricating oil composition.

The lubricating oil composition of the present invention will containfrom about 0.02 wt % to about 0.7 wt %, preferably from about 0.05 wt %to about 0.5 wt %, and more preferably from about 0.1 wt % to about 0.3wt % of a zinc primary diaryl dithiophosphate, based on the total weightof the lubricating oil composition.

In a preferred embodiment, the lubricating oil composition of thepresent invention will have a phosphorus content preferably less thanabout 0.05 wt %, based on the total weight of the lubricating oilcomposition.

In another embodiment, the lubricating oil composition of the presentinvention will further have a sulfur content less than about 0.5 wt %and, preferably less than about 0.2 wt %, based on the total weight ofthe lubricating oil composition and the total sulfated ash content inthe lubricating oil composition of the present invention is less thanabout 1.2 wt %, preferably, less than about 1.0 wt %, and morepreferably less than about 0.8 wt %, based on the total weight of thelubricating oil composition.

Other Additive Components

The following additive components are examples of components that can befavorably employed in combination with the lubricating additive of thepresent invention. These examples of additives are provided toillustrate the present invention, but they are not intended to limit it.

-   -   (A) Detergents are additives designed to hold the        acid-neutralizing compounds in solution in the oil. They are        usually alkaline and react with the strong acids (sulfuric and        nitric) which form during the combustion of the fuel and which        would cause corrosion to the engine parts if left unchecked.        Examples are carboxylates, sulfurized or unsulfurized alkyl or        alkenyl phenates, alkyl or alkenyl aromatic sulfonates,        sulfurized or unsulfurized metal salts of multi-hydroxy alkyl or        alkenyl aromatic compounds, alkyl or alkenyl hydroxy aromatic        sulfonates, sulfurized or unsulfurized alkyl or alkenyl        naphthenates, metal salts of alkanoic acids, metal salts of an        alkyl or alkenyl multiacids and chemical and physical mixtures        thereof.    -   (B) Dispersants are additives that keep soot and combustion        products in suspension in the body of the oil and therefore        prevent deposition as sludge or lacquer. Typically, the ashless        dispersants are nitrogen-containing dispersants formed by        reacting alkenyl succinic acid anhydride with an amine. Examples        are alkenyl succinimides, alkenyl succinimides modified with        other organic compounds, e.g., ethylene carbonating        post-treatment and alkenyl succinimides modified with boric        acid, polysuccinimides, alkenyl succinic ester.    -   (C) Oxidation Inhibitors:    -   1) Phenol type (phenolic) oxidation inhibitors:        4,4′-methylenebis (2,6-di-tert-butylphenol),        4,4′-bis(2,6-di-tert-butylphenol),        4,4′-bis(2-methyl-6-tert-butylphenol),        2,2′-methylenebis(4-methyl-6-tert-butyl-phenol),        4,4′-butyldienebis(3-methyl-6-tert-butylphenol),        4,4′-isopropylidenebis(2,6-di-tert-butylphenol),        2,2′-methylenebis(4-methyl-6-nonylphenol),        2,2′-isobutyldiene-bis(4,5-dimethylphenol),        2,2′-methylenebis(4-methyl-6-cyclohexylphenol),        2,6-di-tert-butyl-4-methylphenol,        2,6-di-tert-butyl-4-ethylphenol,        2,4-dimethyl-6-tert-butyl-phenol,        2,6-di-tert-α-dimethylamino-p-cresol,        dimethylaminomethylphenol),        4,4′-thiobis(2-methyl-6-tert-butylphenol),        2,2′-thiobis(4-methyl-6-tert-butylphenol),        bis(3-methyl-4-hydroxy-5-tert-butylbenzyl)-sulfide and        bis(3,5-di-tert-butyl-4-hydroxybenzyl).    -   2) Diphenylamine type oxidation inhibitor: alkylated        diphenylamine, phenyl-α-naphthylamine and alkylated        α-naphthylamine.    -   3) Other types: metal dithiocarbamate (e.g., zinc        dithiocarbamate) and methylenebis(dibutyldithiocarbamate).    -   (D) Rust Inhibitors (Anti-rust agents):    -   1) Nonionic polyoxyethylene surface active agents:        polyoxyethylene lauryl ether, polyoxyethylene higher alcohol        ether, polyoxyethylene nonylphenyl ether, polyoxyethylene        octylphenyl ether, polyoxyethylene octyl stearyl ether,        polyoxyethylene oleyl ether, polyoxyethylene sorbitol        monostearate, polyoxyethylene sorbitol mono-oleate and        polyethylene glycol monooleate.    -   2) Other compounds: stearic acid and other fatty acids,        dicarboxylic acids, metal soaps, fatty acid amine salts, metal        salts of heavy sulfonic acid, partial carboxylic acid ester of        polyhydric alcohol and phosphoric ester.    -   (E) Demulsifiers: addition product of alkylphenol and        ethyleneoxide, polyoxyethylene alkyl ether and polyoxyethylene        sorbitane ester.    -   (F) Extreme pressure agents (EP agents): sulfurized oils,        diphenyl sulfide, methyl trichlorostearate, chlorinated        naphthalene, benzyl iodide, fluoroalkylpolysiloxane and lead        naphthenate.    -   (G) Friction modifiers: fatty alcohol, fatty acid, amine,        borated ester and other esters.    -   (H) Multifunctional additives: sulfurized oxymolybdenum        dithiocarbamate, sulfurized oxymolybdenum organo        phosphorodithioate, oxymolybdenum monoglyceride, oxymolybdenum        diethylate amide, amine-molybdenum complex compound and        sulfur-containing molybdenum complex compound    -   (I) Viscosity Index improvers (VII): polymethacrylate type        polymers, ethylene-propylene copolymers, styrene-isoprene        copolymers, hydrogenated styrene-isoprene copolymers,        hydrogenated star-branched polyisoprene, polyisobutylene,        hydrogenated star-branched styrene-isoprene copolymer and        dispersant type viscosity index improvers.    -   (J) Pour point depressants: polymethyl methacrylates,        alkylmethacrylates and dialkyl fumarate-vinyl acetate        copolymers.    -   (K) Foam Inhibitors: alkyl methacrylate polymers and dimethyl        silicone polymers.

EXAMPLES

The present invention will be further illustrated by the followingexamples, which set forth particularly advantageous method embodiments.While the Examples are provided to illustrate the present invention,they are not intended to limit it.

Example 1

The low phosphorus lubricating oil composition of the present inventionwas prepared by blending a 0.78 wt % mixture of zincbis(O,O′-di-(2-ethyl-1-hexyl) dithiophosphate (0.24 wt %, primary), zincbis(O,O′-di-(2-butyl/4-methyl-2-pentyl) dithiophosphate (0.15 wt %,secondary) and zinc bis(O,O′-di-(dodecylphenyl) dithiophosphate (0.39 wt%, aryl) with a Group II base oil of lubricating viscosity. The ratio ofzinc bis(O,O′-di(2-ethyl-1-hexyl) dithiophosphate to zincbis(O,O′-di-(2-butyl/4-methyl-2-pentyl) dithiophosphate was about 1:1,based on the phosphorus content. The ratio of the mixture of zincbis(O,O′-di(2-ethyl-1-hexyl) dithiophosphate and zincbis(O,O′-di-(2-butyl/4-methyl-2-pentyl) dithiophosphate to zincbis(O,O′-di-(dodecylphenyl) dithiophosphate was about 2:1, based on thephosphorus content. The resulting ratio of the throe-way mixture of zincbis(O,O′-di(2-ethyl-1-hexyl) dithiophosphate to zincbis(O,O′-di-(2-butyl/4-methyl-2-pentyl) dithiophosphate to zincbis(O,O′-di-(dodecylphenyl) dithiophosphate was 1:1:1, based on thephosphorus content. The wt % of phosphorus in the prepared lubricatingoil composition was less than about 0.06 wt % based on the total weightof the lubricating oil composition. Further, the sulfur content andsulfated ash content were 0.2 wt % and 0.8 wt %, respectively, based onthe total weight of the lubricating oil composition. The wt % balance ofthe lubricating oil composition containing a 1200 molecular weight (MW)isobutylene bis-succinimide dispersant, a 2300 MW isobutylenebis-succinimide dispersant, a neutral sulfonate detergent, an overbasedcalcium phenate, a molybdenum oxidation inhibitor, diphenylamineoxidation inhibitor, a phenolic oxidation inhibitor, anti-foam agent,pour point depressant and a viscosity index improver to complete the 100wt % lubricating oil composition.

Comparative Example A

Comparative Example A was prepared according to Example 1 except onlyabout 1.16 wt % aryl zinc bis(O,O′-di-(dodecylphenyl) dithiophosphatewas added, instead of the mixture of zinc bis(O,O′-di-(2-ethyl-1-hexyl)dithiophosphate, zinc bis(O,O′-di-(2-butyl/4-methyl-2-pentyl)dithiophosphate and zinc bis(O,O′-di-(dodecylphenyl) dithiophosphate.

Comparative Example B

Comparative Example B was prepared according to Example 1 except onlyabout 0.46 wt % zinc bis(O,O′-di-(2-butyl/4-methyl-2-pentyl)dithiophosphate was added, instead of the mixture of zincbis(O,O′-di-(2-ethyl-1-hexyl) dithiophosphate, zincbis(O,O′-di-(2-butyl/4-methyl-2-pentyl) dithiophosphate and zincbis(O,O′-di-(dodecylphenyl) dithiophosphate.

Comparative Example C

Comparative Example C was prepared according to Example 1 except onlyabout 0.71 wt % zinc bis(O,O′-di-(2-ethyl-1-hexyl) dithiophosphate wasadded, instead of the mixture of zinc bis(O,O′-di-(2-ethyl-1-hexyl)dithiophosphate, zinc bis(O,O′-di-(2-butyl/4-methyl-2-pentyl)dithiophosphate and zinc bis(O,O′-di-(dodecylphenyl) dithiophosphate.

Comparative Example D

Comparative Example D was prepared according to Example 1 except about0.81 wt % of a mixture of zinc bis (O,O′-di-(2-butyl/4-methyl-2-pentyl)dithiophosphate and zinc his (O,O′-di-(dodecylphenyl) dithiophosphate inabout a 1:1 ratio were added, instead of the mixture of zincbis(O,O′-di-(2-ethyl-1-hexyl) dithiophosphate, zincbis(O,O″-di-(2-butyl/4-methyl-2-pentyl) dithiophosphate and zincbis(O,O′-di-(dodecylphenyl) dithiophosphate.

Comparative Example E

Comparative Example E was prepared according to Example 1 except about0.94 wt % of a mixture of zinc bis(O,O′-di-(2-ethyl-1-hexyl)dithiophosphate and zinc bis(O,O′-di-(dodecylphenyl) dithiophosphate inabout a 1:1 ratio were added, instead of the mixture of zincbis(O,O′-di-(2-ethyl-1-hexyl) dithiophosphate, zincbis(O,O′-di-(2-butyl/4-methyl-2-pentyl) dithiophosphate and zincbis(O,O′-di-(dodecylphenyl) dithiophosphate.

Comparative Example F

Comparative Example F was prepared according to Example 1 except about0.59 wt % of a mixture of zinc bis(O,O′-di-(2-ethyl-1-hexyl)dithiophosphate and zinc bis(O,O′-di-(2-butyl/4-methyl-2-pentyl)dithiophosphate in about a 1:1 ratio were added, instead of the mixtureof zinc bis(O,O′-di-(2-ethyl-1-hexyl) dithiophosphate, zincbis(O,O′-di-(2-butyl/4-methyl-2-pentyl) dithiophosphate and zincbis(O,O′-di-(dodecylphenyl) dithiophosphate.

Each formulation according to Example 1 and Comparative Example A-F weretested for lead corrosion using the High Temperature Corrosion BenchTest (HTCBT) (ASTM D6594) which is an industry standard bench test tomeasure corrosion performance of a motor oil. Briefly, four metalspecimens of copper, lead, tin, and phosphor bronze are immersed in ameasured amount of engine oil. The oil, at an elevated temperature, isblown with air for a period of time. When the test is completed, thelead specimen and the stressed oil are examined to detect corrosion andcorrosion products, respectively. A reference oil is tested with eachgroup of tests to verify test acceptability.

The test results are summarized in Table II.

TABLE II HTCBT Results Example Comparative Examples 1 A B C D E F Lead,48.4 113.4 93.4 305 64.6 87.3 99.4 ppm

These results demonstrate that the low phosphorus lubricating oncomposition of the present invention (Example 1) containing a mixture ofzinc primary dialkyl dithiophosphate, zinc secondary dialkyldithiophosphate and zinc diaryl dithiophosphate in a 1.1:1 ratio, andwherein the phosphorus content of the lubricating oil composition isless than 0.06 wt %, provides excellent lead corrosion performance whencompared to the comparative examples not having a mixture of all threedithiophosphates. The amount of lead corrosion is significantly reducedby the lubricating oil composition of the present invention.

What is claimed is:
 1. A lubricating oil composition comprising a majoramount of base oil of lubricating viscosity and a minor amount of amixture of a zinc primary dialkyl dithiophosphate a zinc secondarydialkyl dithiophosphate and a zinc diaryl dithiophosphate wherein therespective ratio, based on the phosphorus content, of the zinc primarydialkyl dithiophosphate to zinc secondary dialkyl dithiophosphate isfrom about 2:1 to about 1:2 and the ratio of the mixture of zinc primarydialkyl dithiophosphate and zinc secondary dialkyl dithiophosphate tozinc diaryl dithiophosphate is from about 6:1 to about 1:1 and whereinthe lubricating oil composition has less than about 0.06 wt % totalphosphorus content, based on the total weight of the lubricating oilcomposition.
 2. The lubricating oil composition according to claim 1,wherein the minor amount of the mixture of zinc primary dialkyldithiophosphate, zinc secondary dialkyl dithiophosphate and zinc diaryldithiophosphate is from about 0.1 wt % to about 1.5 wt %, based on thetotal weight of the lubricating oil compression.
 3. The lubricating oilcomposition according to claim 1, wherein the minor amount of themixture of zinc primary dialkyl dithiophosphate, zinc secondary dialkyldithiophosphate and zinc diaryl dithiophosphate is from about 0.3 wt %to about 1.2 wt % based on the total weight of the lubricating oilcomposition.
 4. The lubricating oil composition according to claim 1,wherein the minor amount of the mixture of zinc primary dialkyldithiophosphate, zinc secondary dialkyl dithiophosphate and zinc diaryldithiophosphate is from about 0.5 wt % to about 1.0 wt %, based on thetotal weight of the lubricating oil composition.
 5. The lubricating oilcomposition according to claim 1, wherein the mixture contains fromabout 0.05 wt % to about 1.2 wt % zinc primary dialkyl dithiophosphate,from about 0.05 wt % to about 1.2 wt % zinc secondary dialkyldithiophosphate and from about 0.02 wt % to about 0.7 wt % zinc diaryldithiophosphate, based on the total weight of the lubricating oilcomposition.
 6. The lubricating oil composition according to claim 5,wherein the mixture contains from about 0.1 wt % to about 0.7 wt % zincprimary dialkyl dithiophosphate, from about 0.1 wt % to about 0.7 wt %zinc secondary dialkyl dithiophosphate and from about 0.05 wt % to about0.5 wt % zinc diaryl dithiophosphate, based on the total weight of thelubricating oil composition.
 7. The lubricating oil compositionaccording to claim 6, wherein the mixture contains from about 0.2 wt %to about 0.5 wt % zinc primary dialkyl dithiophosphate, from 0.2 wt % toabout 0.5 wt % zinc secondary dialkyl dithiophosphate and from about 0.1wt % to about 0.3 wt % zinc diaryl dithiophosphate, based on the totalweight of the lubricating oil composition.
 8. The lubricating oilcomposition according to claim 1, wherein the primary alkyl group of thezinc primary dialkyl dithiophosphate has from about C₁ to about C₁₃carbon atoms.
 9. The lubricating oil composition according to claim 8,wherein the primary alkyl group of the zinc primary dialkyldithiophosphate has from about C₃ to about C₁₀ carbon atoms.
 10. Thelubricating oil composition according to claim 9, wherein the primaryalkyl group of the zinc primary dialkyl dithiophosphate has from aboutC₆ to about C₈ carbon atoms.
 11. The lubricating oil compositionaccording to claim 1, wherein the secondary alkyl group of the zincsecondary dialkyl dithiophosphate has from about C₃ to about C₁₃ carbonatoms.
 12. The lubricating oil composition according to claim 11,wherein the secondary alkyl group of the zinc secondary dialkyldithiophosphate has from about C₃ to about C₈ carbon atoms.
 13. Thelubricating oil composition according to claim 12, wherein the secondaryalkyl group of the zinc secondary dialkyl dithiophosphate has from aboutC₃ to about C₆ carbon atoms.
 14. The lubricating oil compositionaccording to claim 1, wherein the aryl. group of the zinc diaryldithiophosphate has from about C₆ to about C₃₀ carbon atoms.
 15. Thelubricating oil composition according to claim 14, wherein the arylgroup of the zinc diaryl dithiophosphate has from about C₈ to about C₂₄carbon atoms.
 16. The lubricating oil composition according to claim 15wherein the aryl group of the zinc diaryl dithiophosphate has from aboutC₆ to about C₂₀ carbon atoms.
 17. The lubricating oil compositionaccording to claim 1, wherein the respective ratio of zinc primarydithiophosphate to zinc secondary dialkyl dithiophosphate is a rangefrom about 3:2 to about 2:3, based on the phosphorus content.
 18. Thelubricating oil composition according to claim 17, wherein therespective ratio of zinc primary dithiophosphate to secondary dialkyldithiophosphate is about 1:1, based on the phosphorus content.
 19. Thelubricating oil composition according to claim 1, wherein the respectiveratio of the mixture of zinc primary dialkyl dithiophosphate and zincsecondary dialkyl dithiophosphate to zinc diaryl dithiophosphate is arange from about 4:1 to about 1:1, based on the phosphorus content. 20.The lubricating oil composition according to claim 19, wherein therespective ratio of the mixture of zinc primary dialkyl dithiophosphateand zinc secondary dialkyl dithiophosphate to zinc diaryldithiophosphate is about 2:1 based on the phosphorus content.
 21. Thelubricating oil composition according to claim 1, wherein the respectiveratio of the mixture of zinc primary dialkyl dithiophosphate to zincsecondary dialkyl dithiophosphate to zinc diaryl dithiophosphate is1:1:1, based on the phosphorus content.
 22. The lubricating oilcomposition according to claim 1, wherein the total phosphorus contentof the lubricating oil composition in less than 0.05 wt %, based on thetotal weight of the lubricating oil composition.
 23. The lubricating oilcomposition according to claim 1, wherein the total sulfur content ofthe lubricating oil composition in less than 0.5 wt %, based on thetotal weight of the lubricating oil composition.
 24. The lubricating oilcomposition according to claim 23, wherein the total sulfur content ofthe lubricating oil composition in less than 0.2 wt %, based on thetotal weight of the lubricating oil composition.
 25. The lubricating oilcomposition according to claim 1, wherein the total sulfated ash contentof the lubricating oil composition in less than 1.2 wt %, based on thetotal weight of the lubricating oil composition.
 26. The lubricating oilcomposition according to claim 25, wherein the total sulfated ashcontent of the lubricating oil composition in less than 1.0 wt %, basedon the total weight of the lubricating oil composition.
 27. Thelubricating oil composition according to claim 26, wherein the totalsulfated ash content of the lubricating oil composition in less than 0.8wt % based on the total weight of the lubricating oil composition.
 28. Amethod for improving lead corrosion, said method comprising operating aninternal combustion engine with a lubricating oil composition comprisingit major mount of base oil of lubricating viscosity and a minor amountof a mixture of zinc primary dialkyl dithiophosphate, zinc secondarydialkyl dithiophosphate and zinc diaryl dithiophosphate wherein therespective ratio, based on the phosphorus content, of the zinc primarydialkyl dithiophosphate to zinc secondary dialkyl dithiophosphate isfrom about 2:1 to about 1:2 and the ratio of the mixture of zinc primarydialkyl dithiophosphate and zinc secondary dialkyl dithiophosphate tozinc diaryl dithiophosphate is from about 6:1 to about 1:1 and whereinthe lubricating oil composition has less than about 0.06 wt % totalphosphorus content, based on the total weight of the lubricating oilcomposition.